This invention relates to methods of making large spin-blanks out of pieces of aluminum sheet or plate joined by friction-stir-welding, to provide plate sizes greater than 156 inches in diameter and sheet sizes greater than 139 inches in width. One particular application for such large spin-blanks is the making of large diameter domes for tanks such as liquid fuel rocket tanks.
Advancements in satellites, and the enhancements to telecommunications and other services that sophisticated satellites make possible, have dramatically increased the number of commercial satellites being launched. As each launch is an expensive event, there has been a trend toward increasing the number of features and components on satellites, allowing multiple users to share the costs and benefits of the satellite launch. Increased features have resulted in an increase in the size of these satellites, and consequently the size of the launch vehicles required to carry these satellites into their operational orbit. Generally, satellite launch vehicles are multi-stage rockets, with each stage including its own fuel tank, comprising a cylindrical body having a domed leading end. It is in the manufacture of these larger rocket domes that current manufacturing methods fall short. A number of rocket manufacturers have elected to use spun domes when the blank size is sufficient. The commercial aircraft industry also uses large size spun parts for some if its applications, such as an aircraft engine intake, which comprises a half toroidal shape.
Spun parts, such as rocket domes, have typically been fabricated from single blanks of aluminum alloys that are hot spun over mandrels to form the desired shape. Common mandrel spin forming methods include clamping a blank between a rotatable spindle and a die, or mandrel, corresponding to the shape to be formed. The clamped assembly is then rotated and the blank is heated while a tool, such as a spinning roller, is used to apply pressure, progressively, to a small area of the metal blank, thereby causing the small area to deform in the direction of the pressure. As the metal blank rotates, a circumferential band of the blank is progressively deformed and, by moving the spinning roller in a radial direction with respect to the rotating metal blank, the desired shape is produced.
Traditionally, spun parts have been manufactured by the mandrel spinning process from blanks having a surface area that is greater than or equal to the surface area of the domes to be spun. This process results in a dome having the desired diameter and a substantially constant material thickness. Traditional hot mandrel spinning methods have been effective, as long as the surface area of the necessary blank was smaller than the largest commercially available blank diameter. However, to manufacture spun parts using traditional mandrel spinning techniques would require a circular blank with an outside diameter well in excess of the maximum commercially available blank size. The largest plate mill in the world, an Alcoa mill in Davenport, Iowa., can currently provide plate widths up to 209 inches (531 cm).
Despite these prior art systems, there remains a need in the art for a less costly and more robust way to fabricate spin-blanks larger than 209 inches in diameter for large diameter tank domes and aircraft applications. The cost of developing and producing a wider width mill for the limited production available is not considered a viable possibility. Therefore, a method of joining less expensive standard width sheets or plates is required to provide spin blanks for either traditional spinning or methods such as are described in U.S. Pat. No. 6,199,419 to Shrayer et al., herein expressly incorporated by reference.
Fusion welding has been considered in the past, but has never developed into a production process for a variety of reasons. First, the high strength 7000 series alloys used extensively in the aircraft and aerospace industries are normally considered to be unsuitable for fusion welding. Second, fusion welding is viewed as a high cost process because of the number of weld passes required (for example, 2.5 inch thick 2219 plate would normally require six or more weld passes). Third, there are considerable associated costs incurred in inspecting each pass, as well as great difficulty in controlling the quality and defect level of fusion welds. Fourth, there is considerable potential of tearing the blank during spinning. Shrayer et al. acknowledge, in col. 2, line 67 and col. 3, line 1, the high cost of fusion welded blanks, and thus teach away from such an approach. Thus, for all of the foregoing reasons, fusion welded blanks have not transitioned into production.
Solid-state joining where no melting occurs circumvents the issues associated with fusion welding. However, prior to 1991, this technique was not developed for creating a joint along the length of two butted plates, but rather had been essentially restricted to joining parts that could be rotated and pushed together. A new form of solid state joining or welding, friction-stir welding, was invented by the Welding Institute in England in around 1991, and patented as U.S. Pat. No. 5,460,317 by Thomas et al. This '317 patent is herein expressly incorporated by reference. The solid state joining process taught in the '317 patent is being used in production in the aerospace industry for the joining of fully heat-treated aluminum alloys for making the longitudinal joints of cylindrical sections of tanks for expendable launch vehicles. Friction-stir welding has not, however, heretofore been considered for creating a joint in annealed (O temper) or as-rolled (F temper) aluminum. Current practice is to spin in the annealed temper.
The availability of plate blank sizes greater than 209 inches in diameter would create advantageous manufacturing possibilities for large one-piece spun parts instead of constructing large structures from pieces. This also applies to sheet except that the maximum width available is well under 209 inches. The availability of large blank sizes, particularly for sheet, would permit the use of stretching and spinning, instead of the current practice of bending as the predominant method of metal movement, with greatly advantageous results.